It is generally known to use p-n diodes as Zener diodes in motor vehicle generator systems. The advantages of using such p-n diodes reside, in particular, in that, on the one hand, they have a low reverse current and, on the other hand, are highly rugged. A disadvantage of using such p-n diodes is the high forward voltage UF thereof. At room temperature, the current does not begin to flow until UF=0.7 V. Under normal operating conditions, for example, at a current density of 500 A/cm2, the forward voltage rises to over 1 V. This signifies a considerable loss in the efficiency of a motor vehicle generator.
Schottky diodes are an alternative to p-n diodes. Such Schottky diodes have a significantly lower forward voltage than p-n diodes, for example, 0.5 V to 0.6 V at a high current density of 500 A/cm2. Moreover, as majority carrier components, Shottky diodes offer advantages for a rapid switching operation. Simple Schottky diodes are not particularly suited for motor vehicle generator systems. This is attributable to several important drawbacks of Schottky diodes, in particular to the higher reverse current thereof in comparison to p-n diodes, to the heavy dependency of the reverse current thereof on the reverse voltage, and to the insufficient ruggedness thereof, in particular at high temperatures.
Improved Schottky diodes have also been already provided.
A trench MOS barrier Schottky diode (TMBS) is an improved Schottky diode. FIG. 1 shows a sectional view of this type of known TMBS that is preferably realized in the form of a chip. A TMBS has an n+ substrate 1, an n-epitaxial layer 2, at least two trenches 6 realized in the n-epitaxial layer by etching, a metal layer 4 at the front side of the chip as an anode electrode, a metal layer 5 at the rear side of the chip as a cathode electrode and dielectric layers 7 between trenches 6 and metal layer 4. Dielectric layers 7 are oxide layers, for example. From an electrical standpoint, a TMBS is a combination of an MOS structure, to which metal layer 4, oxide layers 7, and n-epitaxial layer 2 belong, and a Schottky diode, which is formed by the Schottky barrier that is present between metal layer 4 as an anode and n-epitaxial layer 2 as a cathode.
In the forward direction of a TMBS, currents flow through the mesa regions between trenches 6. Trenches 6 are not available for the current flow. Therefore, in the case of a TMBS, the effective surface for the current flow in the forward direction is smaller than in the case of a conventional planar Schottky diode.
An important advantage of a TMBS resides in the reduction of the reverse currents. Both in the case of the MOS structure, as well as in the case of the Schottky diode, space charge zones form in the reverse direction. The space charge zones expand in response to increasing voltage and, at a voltage that is lower than the breakdown voltage of the TMBS, collide in the middle of the region between adjacent trenches 6. This shields the Schottky effects responsible for the high reverse currents, thereby reducing the reverse currents. This shielding effect is heavily dependent on the structural parameters Dt (depth of the trench), Wm (distance between the trenches), Wt (width of the trench), as well as on To (thickness of the oxide layer).
A disadvantage of a TMBS, particularly in the case of a TMBS having a lower barrier height, is the round, respectively soft reverse characteristic. This means that, already well before the actual breakdown, that occurs, for example, at a voltage of 70-80% of the breakdown voltage, the reverse current already rises markedly and is significantly greater than the reverse current at a lower voltage. This substantial reverse current in the pre-breakdown of the Schottky diode induces a substantial power loss, especially at high temperatures, and can lead to thermal instability and to failure of the component as the result of a positive electrical-thermal feedback.
Another improved Schottky diode is a trench Schottky barrier Schottky diode (TSBS). FIG. 2 shows a sectional view of this type of known TSBS that is preferably realized in the form of a chip. A TSBS has an n+ substrate 1, an n-epitaxial layer 2, at least two trenches 6 realized in n-epitaxial layer 2 by etching, and a metal layer 5 at the rear side of the chip as an ohmic contact, respectively cathode electrode. Trenches 6 are first partially filled with a metal 4b having a thickness Dm1 and subsequently covered with a second metal 4a. Second metal 4a fills the remainder of the trenches to a thickness of Dm2. Both metals 4a and 4b at the front side of the chip are used as Schottky contacts, respectively as an anode electrode.
Second metal 4a typically has a lower barrier height than first metal 4b. Therefore, from an electrical standpoint, the TSBS is a combination of two Schottky diodes having different barrier heights: one Schottky diode having a Schottky barrier between metal 4b as an anode and n-epitaxial layer 2 as a cathode, and a second Schottky diode having a Schottky barrier between metal 4a as an anode and n-epitaxial layer 2 as a cathode.
The currents flow in the forward direction at least when the barrier heights of the two metals are distinctly different, mainly to upper metal 4a having the lower barrier, also at the corresponding side walls of the trenches. Therefore, in the case of a TSBS, the effective surface for the current flow in the forward direction can be greater than in the case of a conventional planar Schottky diode.
In the reverse direction, first metal 4b, with the greater barrier height thereof, provides for a large expansion of the space charge zones. The space charge zones expand in response to increasing voltage and, at a voltage that is lower than the breakdown voltage of the TSBS, collide in the middle of the region between adjacent trenches 6. This shields the Schottky effects responsible for the high reverse currents, thereby reducing the reverse currents. This shielding effect is heavily dependent on structural parameters Dt (depth of the trench), Wm (distance between the trenches), Wt (width of the trench), as well as Dm1 (thickness of first metal 4b).
An advantage of a TSBS is the combination of both metals, which allows a certain separation of the structures with respect to the requirements of the forward voltage and shielding behavior. Forward voltage UF and the initial value of reverse current IRO are predominantly influenced by second metal 4a having a lower barrier. The greater the proportion of second metal 4a is, the lower is UF, and all the higher is IRO. On the other hand, first metal 4b having a greater barrier determines the voltage dependency of the reverse current, the breakdown voltage, and the current distribution at high reverse currents. For that reason, the TSBS offers an optimization possibility by combining the two metals. Both thicknesses Dm1 and Dm2, as well as the barrier heights of the two metals may be used as parameters.
As in the case of the TMBS, a disadvantage of the TSBS is the round, respectively soft reverse characteristic.
Another improved Schottky diode is a trench junction barrier Schottky diode (TJBS). A sectional view of a TJBS of this type is shown in FIG. 3. A TJBS has an n+ substrate 1, an n-epitaxial layer 2, at least two trenches 6 etched into n-epitaxial layer 2, a metal layer 4 at the front side of the chip as an anode electrode, and a further metal layer 5 at the rear side of the chip as a cathode electrode. Trenches 6 are filled with p-doped silicon or polysilicon.
From an electrical standpoint, the TJBS is a combination of a p-n diode (p-n transition between the p-doped silicon or polysilicon as an anode and an n-epitaxial layer 2 as a cathode) and a Schottky diode (Schottky battier between metal layer 4 as an anode and n-epitaxial layer 2 as a cathode).
In the forward direction, currents flow only through the Schottky diode. However, due to the lacking lateral p-diffusion, similarly to the TMBS, the effective surface for the current flow in the forward direction in the case of the TJBS is significantly greater than in the case of a conventional junction barrier Schottky diode.
In the reverse direction, the space charge zones expand in response to increasing voltage and, at a voltage that is lower than the breakdown voltage of the TJBS, collide in the middle of the region between adjacent trenches 6. This shields the Schottky effect responsible for the high reverse currents, thereby reducing the reverse currents. This shielding effect is heavily dependent on structural parameters Dt (depth of the trench), Wn (distance between the trenches), as well as Wp (width of the trench).
The TJBS can provide a substantial ruggedness due to the clamping function thereof when the breakdown voltage of p-n diode 8-2 is dimensioned to be lower than that of Schottky diode 4-2, and the breakdown takes place at the bottom of the trenches. In breakdown operation, the reverse current flows then only through the p-n junction. Thus, forward direction and reverse direction are geometrically separate. Therefore, the ruggedness of the TJBS is similar to that of a p-n diode.
If the breakdown voltage of p-n diode 8-2 is significantly lower than that of Schottky diode 4-2, then the TJBS can significantly reduce the voltage dependency of the leakage currents caused by the Schottky effect, and exhibit a square reverse characteristic.
German Published Patent Application No. 10 2004 053 761 describes a TJBS of this kind having an integrated p-n diode, as well as a method for manufacturing such a TJBS.